Many buildings are fitted with alarm systems that sense the presence of intruders through various types of sensor switches. Simple sensor switches comprise a make-or-break switch connected to, for instance, a door or window, in a building. When an intruder opens the door or the window, contact between the make-or-break switch is broken and this causes an alarm to sound. Unfortunately, this type of security system requires many wires to be run around the building between the windows and the doors to the alarm and also, the alarm only sounds after the door or window has been broken into. More elaborate security systems are available which employ acoustic detectors as sensor switches. The acoustic detector detects sound waves generated within the room. If an intruder enters the room, the detector senses a frequency shift in the sound waves received due to the Doppler effect and when a frequency shift is detected, the acoustic detector causes an alarm to sound. Similar systems are available which employ infra-red detectors rather than acoustic detectors. These systems suffer from the disadvantages that they are relatively complex, and again are only activated after an intruder has entered the room.
Electronic devices have been proposed that to some extent overcome the above mentioned problems in that they cause an alarm to sound before a-n intruder has entered the room. A portable electronic device which can sense an intruder attempting to open a door, before the door is actually opened, has been proposed. This device comprises a loop shaped antenna which is placed onto a doorknob of the door and an electronic circuit connected to the antenna sensitive to changes in the circuit formed by the antenna due to capacitive coupling between the antenna and objects in contact with the doorknob. When an intruder attempts to turn the doorknob, the electronic circuit detects a change in capacitive coupling and causes an alarm to sound before the door is opened. Unfortunately, this device is still relatively complex and in order that it operates reliably the device must be securely mounted to the door by fastening the antenna tightly around the doorknob or fixing it to the doorknob with velcro or a suction cup. Thus, although this device is portable, it is really only suitable as a security device against potential intruders to a room.
It would be desirable to have a device that is simple in construction and provides a reliable alarm signal when the presence of a potential intruder is detected. Also, such a device would ideally provide a truly portable alarm which could be used as security against theft of objects outside of rooms, such as baggage left on a beach or goods temporarily left in a shop, for instance.